Reading a file name with null bytes fails, but has no err.code

As reported in https://github.com/tmpvar/jsdom/pull/1003, if you do `fs.readFile("<div>\0</div>", "utf-8", function (err) { })` then `err` will only be detectable via `err.toString() == 'Error: Path must be a string without null bytes.'`. Having a `.code` would be better.
